A great many vehicles, such as snowmobiles, have a variable speed belt transmission driving system (sometimes referred to as torque converters). In such a system, there is a drive or driving clutch with a movable conical faced sheave and a fixed conical faced sheave, a driven clutch with a movable conical faced sheave and a fixed conical faced sheave, a transmission belt extending between each pair of sheaves coupling the driving and driven clutches, a speed responsive or displacement means such as a plurality of cams, flyweights or other means which are operatively associated with the movable sheave of the driving clutch and a biasing or resistance means in the driving and driven clutch to oppose the speed responsive or displacement means.
The purpose of the driving clutch is to control the speed of the engine in all gear ratios as the transmission changes gears. There is a biasing or resistance means in the driving clutch that works against the speed responsive or displacement means associated with the movable sheave. The driving clutch can be adjusted to achieve a predetermined desired engine speed by modifying the speed responsive or displacement means or, in accordance with this invention, by modifying the biasing or resistance means.
The purpose of the driven clutch is to provide enough side pressure on the transmission belt to allow power to be transmitted from the driving clutch to the driven clutch without the belt slipping. The side pressure on the belt has a lot to do with power loss and backshifting. The higher the belt pressure, the quicker the backshifting but the higher the power loss, also. Therefore, the driven clutch biasing means is selected to be a compromise between quick backshifting and low power loss. High belt side pressure also results in increased belt wear and shorter belt life.
In operation, what happens is that at low rotational speeds of the driving clutch, the fixed and movable sheaves of the driving clutch are forced apart by a biasing or resistance means (e.g., compression spring) parallel to the centerline of a drive shaft running between the fixed and axially movable sheaves, thus enabling the transmission belt to ride near the bottom of the driving clutch thereby creating a small diameter drive wheel. Correspondingly, the sheaves of the driven clutch are forced close together by a biasing means parallel to the centerline of a drive shaft running between the fixed and movable sheaves, thus causing the transmission belt to operate near the outer edge of the conical faces of the driven clutch sheaves thereby creating a large diameter driven wheel. Therefore, at low speeds a small diameter drive wheel clutch is coupled by the transmission belt to a large diameter driven wheel. This is, in effect, a low gear since it requires many turns of the drive wheel to cause one rotation of the driven wheel.
As the rotational speed of the driving clutch increases in response to increased engine speed, speed responsive or displacement means (i.e., cams or flyweights) operatively associated with the movable sheave of the driving clutch, and opposed by the biasing or resistance means located on or parallel to the centerline of the driving clutch as well as side pressure on the transmission belt caused by the biasing means in the driven clutch, force the movable sheave of the driving clutch closer to the fixed sheave thereby causing the transmission belt to move outward on the conical radius of the drive wheel so as to operate at a greater distance from the center of the driving clutch. The forces on the transmission belt which cause it to move upward along the conical radius of the drive wheel also cause it to move inwardly against the side pressure along the radius of the driven wheel thus forcing the movable sheave of the driven clutch away from its associated fixed sheave. Thus higher rotational speeds of the driving clutch cause the transmission belt to effectively form a large diameter drive wheel and a small diameter driven wheel. This is, essentially, a high gear since it enables one complete rotation of the drive wheel to cause several complete rotations of the driven wheel. This means that such a transmission belt drive system has a built-in capability of effectively switching from a low gear to a higher gear as the rotational speed of the drive wheel is increased.
However, there is an inherent disadvantage to this type of system in which the gear ratio is automatically changed with an increase or decrease in rotational speed of the drive clutch. This disadvantage exists because the gear ratio change can occur at a time when constant speed is desired such as when travelling down a road or trail or the gear ratio can remain fixed at a time when it should be varied, such as when encountering an increased load like a hill or a turn.
Consider, for example, a snowmobile which utilizes the transmission belt drive system. Normally, a low gear is needed to get the machine moving but after it has commenced moving and the throttle is advanced, the gear ratio begins to change in order to place the vehicle in a higher gear. This is normal operation and no problem occurs.
When it is desired to travel at a constant speed the transmission will operate in a higher gear ratio than is needed if the clutch has been tuned to operate at the top of the power band thereby forcing the operator to drive in a higher gear than is necessary for the given conditions. This results in high power output and poor fuel economy.
Further, assume it is desired to climb a hill or in some manner the load placed upon the vehicle is increased. If the vehicle is travelling at a high rate of speed the driving clutch rotational speed is high and the machine is in a high gear ratio. However, when a vehicle is attempting to climb a hill or move a heavy load under increasing load conditions, it needs a lower gear ratio, therefore such a higher gear ratio is an undesirable situation. That is, at this time greater torque is needed at the driven wheel, not greater speed. In order to achieve greater torque, the transmission must backshift.
Therefore, a particular disadvantage of this type of known system is that the vehicle is slow to backshift, (i.e., downshift) in response to this need for greater torque. The reason being that a compression spring that initially pushes the sheaves of the driving clutch apart and the compression spring that initially pushes the sheaves of the driven clutch together is a constant rate spring that is selected to compromise between a spring that upshifts quickly or downshifts quickly. If the spring rate is high, the transmission will upshift slowly but downshift quickly. If the spring rate is low, the transmission will upshift quickly but downshift slowly.
There is a long-felt need for a device which will overcome these problems and allow the driving system to upshift quickly when a higher speed is desired and downshift quickly when more torque is needed in addition to allowing the engine to be operated at either its most fuel efficient speed setting or its most powerful speed setting.
Two-stroke engines that commonly use variable speed transmission belt driving systems operate more efficiently when operated in a power band 94 shown in the speed diagram in FIG. 1. See Olav Aaen, "Clutch Tuning Handbook", Kenosha, Wis. (1993). The speed diagram has the engine speed (rpm) on the vertical axis and the vehicle speed (mph) on the horizontal axis. Line 88 represents the low gear ratio of a typical variable speed transmission. The low gear ratio occurs where the transmission belt is near the bottom of the sheaves of the driving clutch and near the outer edges of the sheaves of the driven clutch. Typically, the low gear ratio is 3:1, whereby the driven clutch rotates one time for every three rotations of the driving clutch.
Line 90 represents the high gear ratio of the transmission. The high gear ratio occurs where the belt is near the outer edges of the driving clutch and near the bottom of the sheaves of the driven clutch. Typically, the high gear ratio in overdrive is 0.75:1, whereby the driven clutch rotates one time for every 3/4 of a rotation of the driving clutch.
Line 92 is a typical shift curve of a two-stroke engine with a power peak at 8250 rpm. The power band 94 is shown by the shaded area. The power band 94 represents a range of engine speeds in which the engine is delivering optimum power. The top 96 of the power band 94 is the power peak of the engine. Operating the engine at its power peak is ideal for performance (high speed) riding. The bottom 98 of the power band 94 is ideal for fuel efficiency because the engine is not creating more horsepower than is needed to move the vehicle at normal cruise speeds.
Two points on the speed diagram that are of particular importance are the engagement speed 100 and the shift-out speed 102. The engagement speed 100 is the engine speed (rpm) required to start the vehicle moving. At the engagement speed 100, the speed responsive or displacement means in the driving clutch overcome the pretension of the biasing or resistance means in the driving clutch and start moving the movable sheave toward the fixed sheave until enough force is exerted on the belt to start the vehicle moving. After the driving sheaves have gripped the belt without slipping the vehicle will accelerate along the low ratio line 88. While the vehicle speed is increasing in the low gear ratio, the belt remains at the bottom of the driving sheaves.
The second important point occurs when the engine speed has built up enough centrifugal force in the speed responsive or displacement means to overcome both the pressure of the biasing or resistance means in the driving clutch and the side pressure on the belt by the biasing means in the driven clutch, the belt will move out on the driving sheaves, move in on the sheaves of the driven clutch, and the ratio of the transmission will change (i.e., shift up). This is the shift-out speed 102 and should be within the power band 94.
Typically, the shift curve 92 is in the power band 94 of the engine and is essentially straight from the shift-out speed 102 to the high ratio line 90. This means the engine speed is held constant in the area where the engine is delivering optimum power while the transmission ratio is changing (i.e., the transmission is up-shifting) and thus the vehicle speed is increasing.
As noted before, the purpose of the driven clutch is to provide enough side pressure on the belt to allow power to be transmitted to the ground to move the vehicle. However, too much side pressure results in reduced belt life, power loss, and decreased efficiency. In operation, more side pressure is needed in low ratio (about twice as much) than is needed in high ratio therefore typical driven clutches have a torsion spring and torque feed back ramp design that produces such a desired effect in the belt side pressure.
An object of the present invention is to match the biasing or resistance means of the driving clutch along with the belt pressure created by the driven clutch to the speed responsive or displacement means so that the engine speed is held at or near bottom 98 of the power band 94 for maximum engine efficiency (i.e., fuel efficiency), reduced noise and reduced vibration from the low ratio line 88 to predetermined speed 107. Predetermined speed 107 is selected to correspond with a maximum normal cruise speed. At speed 107, it is an object of the present invention to match the biasing or resistance means of the driving clutch along with the belt pressure created by the driven clutch to the speed responsive or displacement means so that the engine speed is held at or near top 96 of power band 94 for performance speeds from speed 107 to high ratio line 90.
It is commonly accepted to use a single spring with a constant spring rate as the biasing or resistance means in both the drive and driven clutches, as shown in U.S. Pat. No. 3,362,242 to Watkins and U.S. Pat. No. 3,709,052 to Lassanske.
This is a simple means to offset the initial axial displacement of the sheaves but the disadvantage to this approach is that the linear spring rate provided by one spring does not take into account the parasite drag of the vehicle or the varying terrain that the vehicle might encounter. Parasite drag is the drag on the vehicle caused by wind resistance, sliding resistance, etc. The parasite drag on a vehicle increases non-linearly as the speed of the vehicle increases. Take for example, when a snowmobile is moving at a relatively low speed, say 40 miles per hour, it requires approximately 25 horsepower. But when the snowmobile is moving twice as fast (i.e., 80 miles per hour), it requires approximately 90 horsepower. It would be desirable to have a biasing means that had a variable spring rate so that as the belt rises up on the drive wheel, the movable sheave would be slowed in its movement by increased axial resistance so that more torque would be delivered at the driven wheel to compensate for the parasite drag. On the other hand, when drag is low the engine speed (rpm) should be low so as to maximize efficiency while still optimizing the gear ratio of the shift-out period.
In addition, two-stroke engines that commonly use such transmission belt driving systems operate more efficiently when operated within the power band discussed above. Therefore, it is also desirable to have a system that allows the engine to operate within this band (i.e., operate at a relatively constant speed), while allowing the output of the system at the driven clutch to vary so as to match the needs of either greater torque or increased speed, as discussed above.
U.S. Pat. No. 3,945,964 to Adams, discloses a variable rate biasing means consisting of two helical springs. However, Adams is directed to achieving a constant output system where the engine operates over a range of speeds so that the speed of the driven wheel will stay constant. In Adams, the springs are arranged to provide a non-linear rate of axial displacement of the movable sheave of the driving clutch and consequently a non-linear rate of speed ratio change.
The non-linear rate of axial displacement of the moveable sheave from the fixed sheave is accomplished by providing sequentially operating springs of different spring rates (i.e., a light spring and a heavy spring). Initially, the heavy spring is preloaded (i.e., partially compressed) between two washers and the light spring is located between a movable sheave on the driving clutch and one of the washers. In operation, the lighter spring is compressed as the speed of the driving clutch is increased and deflects to produce a first rate of axial displacement, then at a predetermined point the light spring is stopped from further compression when a shoulder on the movable sheave contacts the washer between the light spring and the heavy spring. Then the heavy spring will independently start to deflect, or compress, giving a second rate of axial displacement. Adams does not show the use of two springs initially acting simultaneously, before a heavy spring is stopped from deflecting.
It is desirable in variable speed belt transmissions to have a variable rate biasing or resistance means that has a biasing or resistance means in which the first rate is relatively low and then at a predetermined point the rate changes to a higher second rate relatively so as to control the back shift or upshift of the transmission.